As fossil fuels become more scarce, the energy industry has developed more sophisticated techniques for extracting fuels that were previously too difficult or expensive to extract. One such technique is to inject steam into an oil-bearing formation to free up and reduce the viscosity of the oil. Several techniques for steam injection presently exist, and are often collectively referred to as “Thermal Enhanced Oil Recovery,” or “Thermal EOR.” Representative steam injection techniques include cyclic, steamflood, steam-assisted gravity drainage (SAGD), and other strategies using vertical and/or horizontal injection wells, or a combination of such wells, along with continuous, variable-rate, and/or intermittent steam injection in each well.
One representative system for forming steam for steam injection is a fuel-fired boiler, having a once-through configuration or a recirculating configuration. Other steam generating systems include heat recovery steam generators, operating in a continuous mode. Thermal EOR operations often produce steam 24 hours per day, over a period ranging from many days to many years. During the period of continuous steam injection, steam pressures and flow rates may be roughly constant in some cases, may vary within a predetermined limited range in other cases, or may vary more widely according to a predetermined plan. However, it is generally undesirable to shut down and start the steam generating equipment (e.g., on a daily basis) because such cycles increase equipment maintenance associated with thermal cycling, and the potential for increased corrosion during idle or standby periods.
Another representative steam generator is a solar steam generator, which can augment or replace the fuel-fired boilers. Solar steam generators can reduce fuel use, reduce operations costs, reduce air emissions, and/or increase oil production in thermal recovery projects. However, such systems collect energy intermittently due to day/night patterns of available solar radiation, with energy production rates varying according to factors which include the variation of incoming solar radiation. Due to the above-described demand for continuous steam, many or most thermal recovery projects which incorporate solar steam generators will also incorporate one or more fuel-fired steam generators, which combust liquid or gaseous fuels as a supplemental source of heat to form steam from an incoming liquid water stream. However, such combinations of solar and fuel-fired boilers are typically inefficient, e.g., because they include many additional components, waste energy, and/or have excess or redundant capacity. Accordingly, there remains a need for high-efficiency solar steam generation systems.